This invention relates to a highly densified composite shaped article and a method of making the same from powder material and, more particularly, to such a shaped article having a unique combination of magnetic and physical properties made from powder materials by a unique process.
Hitherto, there have been available alloys having good magnetic permeability, which could be readily shaped or formed, as for example, into relatively thin strip for use in making laminations or shields, but such materials were mechanically soft and, when used to make such products as laminated recorder heads or recorder head shields, were subject to excessive wear. On the other hand, other magnetic materials having physical hardness for good wear resistance and which also had good permeability were generally brittle and difficult to shape or form.
Numerous attempts have been made to provide products combining good magnetic properties with hardness or wear resistance using powder starting materials and techniques, but they have left much to be desired. For example, Gabriel et al U.S. Pat. No. 3,814,598 granted June 4, 1974 relates to a powder metallurgy method of producing a hot consolidated ferrous alloy magnetic pole piece from a ferrous alloy powder containing from about 2 to 12% silicon, 2 to 12% aluminum, 2000 to 8000 ppm oxygen and the balance iron in which the oxygen is added by thermally oxidizing the silicon-aluminum-iron alloy. Alexander et al U.S. Pat. No. 3,087,234 granted Apr. 30, 1963 relates to iron group metals having submicron particles of refractory oxides dispersed therein consisting of a metallic component selected from the group consisting of iron, cobalt, nickel and alloys thereof with each other and with other metals which form a nonrefractory oxide and having 0.5 to 50% by volume of a refractory metal oxide dispersed therein. The refractory metal oxide is defined in the patent as having a free energy of formation at 1000.degree. C above 60 kilocalories per gram atom of oxygen, having a melting point above 1000.degree. C, and having an average dimension of 5 to 1000 millimicrons. The patent (Col. 3) indicates twenty different oxides, including Al.sub.2 O.sub.3, as being typical single refractory oxides useful in the composition of that patent. Alexander et al point to the difficulties which attend the incorporation of refractory oxides in metal bodies prepared by using the techniques of powder metallurgy and disclose a method in which the metal in an oxidized state is precipitated as a coating on refractory oxide particles having an average size of 5 to 1000 millimicrons, the coating being reduced by heating below the sintering temperature before the whole is sintered.
Ferromagnetically soft alloys are characterized by relatively high magnetic permeability and relatively low coercive force. It has long been recognized that such alloys were highly sensitive to impurities both with regard to their attainable magnetic properties and the processing required to bring out such properties. It is, therefore, critical in the manufacture of articles such as strip, from which magnetic devices are to be fabricated, that such articles be substantially free of impurities and imperfections which would adversely affect the required magnetic properties. In this regard, with respect to articles made from powder, it is to be noted that less than 100% theoretical density indicates residual voids, and whether or not filled with gas such voids, depending upon their size and occurrence, have an undesired effect on magnetic properties such as permeability similar to that of nonmagnetic inclusions of corresponding size. Consequently, for magnetic devices, such articles should have a density which differs, if at all, from 100% theoretical by no more than an insignificant amount. That is, the density of the finished article or the residual voids present therein should leave the article with at least a minimum level of magnetic properties required of such articles.